Field of the Invention
This invention pertains to a method for fabricating a hybrid IC (integrated circuit) substrate and particularly to a method for fabricating electric conductors integrated with high density on an insulator substrate.
Description of the Prior Art
FIG. 1 is a sectional view schematically showing a part of a ceramic circuit substrate disclosed in Japanese Patent Laying-Open No. 112681/1984. In FIG. 1, a tungsten conductor 102 belonging to a first group is formed on a ceramic substrate 101. The tungsten conductor 102 is covered with a nickel film 103 and a palladium film 104. A thick film of a silver-palladium conductor 105 belonging to a second group is electrically connected with the tungsten conductor 102 through the palladium film 104 and the nickel film 103. The nickel film 103 and the palladium film 104 are provided to improve adhesion and conductivity between the tungsten conductor 102 and the silver-palladium conductor 105.
The ceramic circuit substrate of FIG. 1 is fabricated in the following process. First, the tungsten conductor 102 is provided on a green sheet of ceramic and it is baked in a reducing atmosphere at 1600.degree. C., whereby the ceramic substrate 101 to which the tungsten conductor 102 is firmly stuck is formed. The nickel film 103 is formed on the tungsten conductor 102 by electroless plating and the palladium film 104 is formed on the nickel film 103 by electroplating. After that, silver-palladium paste containing a glass component is printed in a predetermined pattern and it is baked in an oxidizing atmosphere at 900.degree. C. As a result, the thick film conductor 105 of the second group fixedly stuck to the ceramic substrate 101 and electrically connected with the. tungsten conductor 102 of the first group is formed.
FIG. 2 is a sectional view schematically showing a part of another ceramic circuit substrate. The circuit substrate of FIG. 2 is similar to that of FIG. 1, except that a thick film resistor 110 of ruthenium oxide is connected to the silver-palladium conductor 105. Although Japanese Patent Publication No. 112681/1984 does not mention a resistor of ruthenium oxide, it is well known to print and bake a paste containing ruthenium oxide on a ceramic substrate thereby to form a thick film resistor.
The conventional circuit substrate thus constructed is expensive because noble metallic materials such as silver and palladium need to be used. In addition, the palladium film 104 on the nickel film 103 is applied by electroplating and accordingly the pattern to be plated needs to be connected with a voltage source, which imposes limitation to pattern design. Moreover, the silver-palladium conductor is liable to dissolve into fused solder and it might cause trouble due to silver migration. There is involved another disadvantage that fine patterning is difficult because the silver palladium film is patterned by screen printing.
A method using copper paste instead of silver-palladium paste has been proposed to eliminate the above described disadvantages of the silver-palladium film. However, this method requires an accurate control of an atmosphere for baking of the copper paste and a large quantity of nitrogen is consumed. Accordingly, this method cannot always be said to be economical. Moreover, the copper paste needs to be baked in nitrogen, while the thick film resistor of ruthenium oxide needs to be baked in air, and accordingly if copper paste is used, such a thick film resistor of ruthenium oxide cannot not be used. For this reason, it is necessary to use as the resistor material a non-oxide material such as lanthanum hexaboride. Such material involves various drawbacks in characteristics, processing conditions and the like.